I'm a bit surprised that so many skiers only consider comfort when buying ski boots, obviously they have to be comfortable, but what about performance, longevity and safety. Oversized boots aren't much of an issue if you're a complete beginner, but once you decide to buy your own surely you would want them to fit and function as well as possible.
Most recreational ski boots are sized according to the Mondopoint sizing system, which is defined in ISO 9407:1991. It's based on the mean foot length and width in millimetres, the inner length of the shoe/boot is supposed to be equivalent to the foot length plus 1 cm. Unfortunately the ski boot manufacturers don't stick to it, in the 1990s they started to make the boots longer than the marked size indicates so that they feel comfortable in the shop. On average they are 1-1.5 cm longer than they should be.
The Mondopoint system only covers length and width, the most important area of fit in a ski boot is around the instep.
If they aren't a good fit here they won't work very well and there is very little that can be done to modify the fit in this area. The manufacturers don't give any information about the sizing of this area so the only way to check the fit is to look at how the barefoot fits in the shell.
The design work on ski boots is mainly done at one size, usually size 26.5. The boot manufacturers then send resin models to the toolmaker who scans them into his CAM software and scales them up and down to get the range of sizes required. Humans don't scale like this, small feet and large feet have different proportions.
So without the aid of a reasonably competent boot fitter the odds of getting a pair of well fitting boots are slim, even if you don't have any issues with your feet. It's estimated that that over 75% of Alpine skiers buy ski boots that are between one and two sizes too big
http://www.nordica.com/schweiz/stories/the-seven-most-common-mistakes-skiers-make-when-buying-new-boots/?backstage=1 it's pre-programmed by the manufacturers to drive sales.
This doesn't mean that your boots won't be comfortable if you don't go to a boot fitter. I skied in boots that were miles too big for years and never experienced any discomfort, they just won't work as well as they might. This might not be much of an issue if you only want to slide around a ski resort for a week or two a year, but if you want to progress well fitting ski boots are essential.
I have never paid anything for boot fitting. it's always been included in the price of the boots. In fact since I started buying them from a boot fitter I've saved money as I don't have to replace them as often.
Brucie, ski boot fitters aren't snake oil salesmen, but there are snake oil boot fitting systems such as the Fischer vacuum, Surefoot, Strolz, ErtlRenz and AquaNovo, which curiously involves standing in a tank water! Boot fitters generally try to repair the damage that the snake oil salesmen have caused. The real snake oil salesmen in the skiing equipment business are the sales and marketing people and people who do gear reviews, who for the most part haven't got a clue.